The present invention relates to an optical scanning unit having a scanning laser beam. More specifically, the invention relates to a light intensity control circuit for modulating a scanning laser beam used in an electrophotographic printer.
Conventionally, in an electrophotographic printer, an image is formed on a photoconductive drum by exposing the photoconductive drum to a scanning laser beam. In such a printer, in order to obtain a halftone image where each pixel can have a plurality of gradation levels, the laser beam is modulated so that energy applied to sequential pixels may be different. That is, the intensity of the laser beam and/or the period of time within which each pixel is exposed, may be changed. Conventionally, two of the more common methods for modulating a laser beam are pulse width modulation and amplitude modulation.
In the laser scanning unit, in order to obtain 256 levels of light intensity using pulse width modulation, the minimum exposing time of the laser beam should be set to 220 picoseconds. Practically, it is impossible to control the laser beam to turn ON and OFF within such a short period of time.
If the amplitude modulation method is used, the exposing period is constant, for example, 56 nanosecond. This period is relatively long, and it is possible to control the laser beam to turn ON and OFF over this period. Therefore, in amplitude modulation, the amplitude of the current applied to a laser diode can be varied in order to obtain the desired intensity level. In an electrophotographic printer having 256 intensity levels for the laser beam, 256 current levels are required.
In this system, the maximum intensity of the laser beam or black level, corresponding to a black image, is first established. Then, the other intensity levels are determined based on the black level. However, the characteristic of the laser diode may change below a certain threshold value. If a minimum level (i.e., white level) is below the threshold value, then for low current levels, the output of the laser diode will be similar over a range of current levels. In this case, a sufficient halftone image may not be obtained since the lower intensity range of the laser beam is not reproduced correctly.
Further, the threshold value varies according to temperature. Therefore, the lower levels of the current values may be higher or lower than the threshold value, depending on the temperature. This will result in inconsistent printing if the temperature varies slightly from one scanning line to the next scanning line.